Many saline minerals are recognized as being commercially valuable. For example, trona, borates, potash and sodium chloride are mined commercially. After mining, these minerals need to be beneficiated to remove naturally occurring impurities.
With regard to trona (Na.sub.2 CO.sub.3.NaHCO.sub.3.2H.sub.2 O), high-purity trona is commonly used to make soda ash, which is used in the production of glass and paper. Naturally-occurring trona, or crude trona, is found in large deposits in the western United States, such as in Wyoming and California, and also in Egypt, Kenya, Botswana, Tibet, Venezuela and Turkey. Crude trona ore from Wyoming is typically between about 80% and about 90% trona, with the remaining components including shortite, halite, quartz, dolomite, mudstone, oil shale, kerogen, mica, nahcolite and clay minerals.
The glass and paper making industries generally require soda ash produced from trona having a purity of 99% or more. In order to obtain such a high purity, wet beneficiation processes have been used. Such processes generally involve crushing the crude trona, solubilizing the trona, treating the solution to remove insolubles and organic matter, crystallizing the trona, and drying the trona which may subsequently be calcined to produce soda ash. Alternatively, the crude trona can be calcined to yield crude sodium carbonate, which is then solubilized, treated to remove impurities, crystallized and dried to produce sodium carbonate monohydrate.
Not all industries which use trona require such a highly purified form of trona. For example, certain grades of glass can be produced utilizing trona having less than 97% purity. For this purpose, U.S. Pat. No. 4,341,744 discloses a dry beneficiation process which is less complex and less expensive than the above-described wet beneficiation process. Such a dry beneficiation process generally includes crushing the crude trona, classifying the trona by particle size, electrostatically separating certain impurities, and optionally magnetically separating other impurities. Such a process can yield trona having up to about 95% to 97% purity, depending on the quantity and type of impurities present in the crude trona ore. The lower resulting purities of known dry processes is partially due to the presence of impurities, such as shortite and halite, which have similar electrostatic and magnetic properties as the trona. Further, halite has a similar density as the trona and, therefore, is difficult to remove even utilizing known wet density separation processes.
There are uses for trona, for example, in certain applications in the glass industry, requiring a purity of at least 97%, yet not needing a purity over 99%. The known dry beneficiation processes typically do not consistently produce such a purity. Consequently, these industries generally use trona purified by the more expensive and complex wet beneficiation processes.
Accordingly, it is an object of the present invention to provide a dry process for the beneficiation of saline minerals and in particular, trona, resulting in higher purities than existing dry beneficiation processes and which is simpler and less expensive than known wet beneficiation processes.